Age verification is the check that you are 18 or over, the legal gambling age in Great Britain. Since 7 May 2019, GB-licensed operators must verify age and identity before anyone can gamble, deposit or play free-to-play demo versions of games.
Checks are usually done electronically against official and credit records when you register. If they cannot be completed that way, the operator will ask for documents. Until verification is done you cannot play.
The demo rule surprises some people, but it closes a route by which children could play casino-style games on gambling sites. The same identity checks are also used to apply the right online stake limit for your age. See age verification.